An A4 sheet is 210x297mm, which is 8.27x11.70 in. The ratio of the A4 length to A4 width is exactly the square root of 2 - which is one of the huge advantages of the international ISO paper standards (A4, A5, etc.) over North American formats (letter, legal, etc.) - when folding, say, an A4 sheet in half you get A5 sheet, with exactly the same ratio - the square root of 2, so the proportions (ratio of length over width) are invariant.

So, if you start with a letter size paper (8.5x11 in), cut off a strip along the longer side so that the width becomes even narrower, namely 7.78 in (= 11/sqrt(2)). This way you get a little smaller version of the A4 size paper (7.78x11 in) but with the same proportions.

ok, its a nice model, but i have no idea how to build this. just because the people making this are supposed to have "above average paper airplane making skills" doesn't mean that you should show us one picture for every six folds. Even the first step is not "quite obvious." it's still a nice model, just a bad instructable.